原生动物(protozoan)
任何微小的（常常在显微镜下才能观察到的）单细胞原生生物。它们在大多数土壤、乾净的水和海洋中都可见到。大多数是单独存在的个体，但也有不同的群居物种。原生动物之间以及原生动物和原生生物之间在分类学上的关系时常被改动。最小的已知原生动物是小於2微米的小型血液寄生虫，而最大的则可能有16厘米长，能被肉眼看见。原生动物在形态变化上有差别，但都有一些共同的真核生物特点，如都有油性蛋白质的膜和带膜的液泡及细胞器官（参阅eucaryote）。它们在活动方式、营养和生殖上的差别很大。在原生动物的分类上存在不同的分类系统。主要的门包括肉鞭门（带鞭毛的和带延展细胞质，即伪足）、纤毛门（带纤毛的）、顶覆门、微孢子门和黏体动物门（能产生孢子的）。顶覆门和微孢子门常常被包括在简单的孢子亚门中。常见的原生动物包括腰鞭毛虫、阿米巴和草履虫（参阅Paramecium）。
English version：
protozoan
Any of a group of small (usually microscopic) single-celled protists. They are found worldwide in most soils, fresh water, and oceans. While most are solitary individuals, various colonial forms exist. The taxonomic relationships of protozoans to one another and to other protists continue to be revised. The smallest known protozoans are tiny blood parasites less than 2 microns long; the largest may be 16 mm long and visible to the naked eye. Protozoan shapes vary, but all share such eukaryotic features as lipid-protein membranes and membrane-enclosed vacuoles and organelles (see eukaryote). They show wide variation in modes of movement, nutrition, and reproduction. Various classification systems exist to group the protozoans. The major phyla include Sarcomastigophora (flagellated forms and forms possessing cytoplasmic extensions called pseudopodia), Ciliophora (ciliated forms), and Apicomplexa, Microspora, and Myxozoa (spore-producing forms). Apicomplexa and Microspora are sometimes included in the single phylum Sporozoa. Commonly known protozoans include dinoflagellates, amoebas, and paramecia (see paramecium).